Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/16—Preparation
  • C01B32/162—Preparation characterised by catalysts
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/16—Preparation

Definitions

• the present disclosure relates to a process for the preparation of carbon nanotubes.
• Carbon nanotubes are cylindrical carbon nanostructures having the length-to- diameter ratio of up to 132,000,000: 1. Such characteristic dimensions facilitate these entities to have unusual properties that enable their application in various fields such as electronic, optics, materials science and pharmaceuticals.
• Chemical vapor deposition is one of the methods utilized for the production of CNTs.
• the floating catalyst method and the supported catalyst method are the two variations of s the CVD.
• a metal precursor and a hydrocarbon are vaporized continuously and passed together with carrier gases into a pre-heated tubular furnace.
• the vapor feed undergoes heating (non-isothermal) to the desired temperature due to heat from the tube walls and the metal precursor is converted into a metallic nanoparticle catalyst, on which the hydrocarbon vapor undergoes pyrolysis to grow CNTs.
• both compounds are made to pass through a temperature gradient that generates certain undesirable side products of both these chemicals. The yield is also reduced.
• the supported catalyst is placed inside the hot zone of the furnace and hydrocarbon vapors are passed over, typically, for about 1 hour to obtain CNTs.
• the supported catalyst is maintained in a fluidized state in a reactor to which the supported catalyst and hydrocarbon are continuously fed, while withdrawing the grown CNTs along with unconverted and other hydrocarbons.
• This requires an advance step for catalyst preparation on the support, and the grown CNTs generally have an agglomerated morphology due to CNT growth restricted to the space available in/on/around the support. Also the support material is retained in the product as an additional impurity.
• CNTs Carbon Nano Tubes
• the present disclosure provides a process for the preparation of carbon nanotubes (CNTs), said process comprising the following steps: i. introducing a precursor for a metal catalyst into a heated furnace to permit said precursor to reside in an isothermal zofie of said furnace, to obtain nano-sized metal catalyst particles; ii. controlled feeding, at least once, through at least one inlet port leading to said isothermal zone a hydrocarbon; iii. pyrolyzing said hydrocarbon to obtain pyrolyzed hydrocarbon; iv. feeding a carrier gas into said furnace at, at least one step selected from the group of steps consisting of:
• the hydrocarbon may be fed iteratively through the at least one inlet port.
• the precursor for forming the metal nano-sized catalyst and the hydrocarbon to be contacted with said catalyst are directly introduced at the isothermal zone; thereby avoiding the formation of side products of the metal precursor and the hydrocarbon.
• the present disclosure also provides an apparatus for the preparation of carbon nanotubes (CNTs). BRIEF DESCRIPTION OF THE DRAWING
• Figure 1 illustrates a schematic of an apparatus (100) for the preparation of carbon nanotubes in accordance of the present disclosure, wherein:
• 102 represents the isothermal zone of a furnace
• 106 represents the plurality of inlet ports
• 1 10 represents the outlet; x axis represents the temperature; and y axis represents the length of the furnace.
• Figure 2 (A), (B) and (C) illustrate the transmission electron microscopic images of the CNTs.
• CNTs carbon nanotubes
• other carbonized products such as CNT fibers, strands, mats and composites.
• CNT yield-diminishing pyrolysis of all the hydrocarbons takes place in the entry zone of the tube when the catalyst nanoparticles are yet to be formed and also in the later zones of the tube as the hydrocarbon: nanoparticle ratio is high.
• the present disclosure focuses on reducing the undesirable conversion of hydrocarbons into products other than CNTs.
• the entire feeding of the hydrocarbon is at the isothermal zone of the furnace tube.
• the controlled split/gradual addition of hydrocarbon through several ports along the length of tube or even along the entire tube length is carried out, reducing the undesirable conversion of hydrocarbons into products other than CNTs.
• the heating by mixing with hot mixture flowing through the tube is found to be faster than the heating from tube walls in the entry zone of the tube.
• the distributed hydrocarbon feed avoids unnecessary pyrolysis of the hydrocarbon and its conversion to products other than CNTs. It also enables the utilization of the catalyst effectively increasing the overall conversion, yield and reducing impurity levels in the final product.
• the catalyst is also tested for deactivation after conversion of hydrocarbon in the tube. To check this, only hydrocarbon is fed to the initial catalyst- CNTs formed and deposited in-situ on the tube walls in the hot zone of the furnace and checked for the conversion of every new shot of hydrocarbon vapors into the tube. It is found that the in-situ formed nanoparticle catalyst is active. Further, the absence of support material results in less agglomerated CNT products as well as elimination of the inorganic impurities in the product. Also a separate step and associated hardware for producing supported catalyst is not required. The process of the present disclosure thus helps to reduce the impurity content in the obtained CNTs.
• the process initially includes introducing a precursor for a metal catalyst in an isothermal zone (102) of a heated furnace of an apparatus (as shown in Figure 1) for the preparation of carbon nanotubes (CNTs).
• the apparatus (100) for carrying out the present process will be described subsequently in the specification.
• the precursor gets reduced to a nano-sized metal catalyst.
• the metal nano-sized catalyst of the present disclosure includes but is not limited to non-supported nano- sized metal catalyst.
• the metal component of the precursor for the metal catalyst and the nano-sized metal catalyst is at least one selected from the group consisting of iron, cobalt, nickel, magnesium, rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold.
• the precursor for the metal catalyst is introduced in the isothermal zone of the furnace.
• the precursor may be introduced through a side entry inlet (106) or through the main inlet (108) of the furnace.
• a hydrocarbon is fed in a controlled manner, at least once, through at least one of a plurality of inlet port (106) into the isothermal zone (102) of the heated furnace (104) to pyrolyze the hydrocarbon.
• the hydrocarbon may be fed in a controlled manner iteratively through the inlet port.
• the resultant pyrolyzed hydrocarbon gets deposited on the surface of the already formed nano-sized metal catalyst and generates CNTs.
• the CNTs so generated may be multiple-wall CNTs, single-wall CNTs or double wall CNTs.
• Other compounds like thiophene may also be used in the synthesis of CNTs (particularly required to make single wall carbon nanotubes) along with the hydrocarbon, catalyst precursor and carrier gas.
• the diameter of multiple- wall CNTs may range between 20 to 40nm (as shown in Figure 2(A), 2(B) and 2 (C).
• the temperature in the furnace (104) of the present disclosure can be maintained by different methods such as electrical heating or introducing a pre-heated carrier gas in the furnace.
• Carrier gas may be introduced into the furnace at, at least one step selected from the group of steps consisting of: feeding the carrier gas before feeding the precursor; feeding the carrier gas along with the precursor; feeding the carrier gas after introducing the precursor; feeding the carrier gas after formation of the CNTs; feeding the carrier gas along with the hydrocarbon; and feeding the carrier gas after feeding the hydrocarbon.
• the pre-heated carrier gas includes but is not limited to helium, nitrogen, argon and the like.
• the carrier gas is nitrogen.
• the hydrocarbon of the present disclosure is in the form of vapor as per an embodiment of the present disclosure.
• a typical embodiment of the process of the present disclosure entails iteratively passing the hydrocarbon through the plurality of inlet ports (106) into the isothermal zone (102) of said furnace (104) in a controlled manner. This is because, gradual and iterative exposure of the hydrocarbon completely utilizes the active stock of nano- sized metal catalyst present in the furnace; thereby achieving a higher yield.
• the concentration of the precursor for the metal catalyst is in the range of 0.1 wt % to 4 wt % of the hydrocarbon concentration.
• the furnace is maintained at a temperature ranging from 700 to 1300°C and the flow rate of hydrocarbon is maintained in the range of 20 to lOOsccm. (* seem: standard cubic centimeters per minute)
• the flow rate of the carrier gas is maintained in the range of 100 to 2000sccm.
• the metal precursor for forming the nano-sized metal catalyst and the hydrocarbon to be contacted with the catalyst are directly introduced at the isothermal zone (102) to avoid the formation of side products of the metal precursor and the hydrocarbon, which is a common occurrence with the conventional processes where the chemicals are exposed to a temperature gradient.
• Carbonized products such as CNT fibers, strands and mats can also be prepared in accordance with the present disclosure by processing the CNTs further, in the same apparatus (100), to obtain the desired products.
• the processing may be done using conventional techniques.
• an apparatus (100) for the preparation of carbon nanotubes (CNTs) and carbonized products as shown in Figure 1.
• the apparatus consists of a furnace (104), typically having one or more non-isothermal zones and an isothermal zone (102), adapted to receive a metal precursor and a hydrocarbon.
• the apparatus further includes a main inlet (108) and a plurality of inlet ports (106) located in the isothermal zone of the furnace which are adapted to convey the metal precursor and the hydrocarbon to the furnace for the preparation of CNTs.
• the apparatus (100) of the present disclosure may further include at least one means (such as a side port or main inlet port) for introducing the precursor for the metal catalyst particles in the isothermal zone of the furnace.
• the apparatus also includes at least one outlet (1 10) for releasing the CNTs and carbonized products formed in the apparatus.
• Example 1 Process for the preparation of CNTs in accordance with the present disclosure
• ferrocene metal catalyst precursor
• isothermal zone of the furnace of the present disclosure maintained at 875 °C to generate iron nanoparticles.
• 100 mg of camphor (hydrocarbon) was fed at the isothermal zone of the furnace through at least one inlet port.
• Nitrogen gas carrier gas
• the amount of CNT obtained was 3.8 mg (Case A).
• hydrocarbon (camphor) dosage was reduced to 25 mg and the catalyst content was increased to 4 mg.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Nanotechnology (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Carbon And Carbon Compounds (AREA)

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• 2014
  • 2014-06-20 WO PCT/IN2014/000410 patent/WO2014203280A2/en active Application Filing
  • 2014-06-20 IN IN2089MU2013 patent/IN2013MU02089A/en unknown

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